Skeletal system anatomy and physiology

We all have 206 bones, and together they make up the skeleton which gives the body structure, protects important organs like the brain and heart, and allows muscles to facilitate movement.

Without bones you’d be a shapeless, immobile blob.

Normally, the skeleton can be broken down into the axial and the appendicular skeleton.

The axial skeleton consists of bones located along the vertical axis of your body. It contains 80 bones; 22 bones that make up the skull, 33 vertebrae, 24 ribs, and the sternum.

The appendicular skeleton consists of bones in your limbs as well as the bones that attach the limb bones to the axial skeleton, like the pelvis and the scapulae.

The appendicular skeleton is made up of around 126 bones; 4 bones in both shoulders, 6 bones in the arms, 54 bones in the hands, 2 hip bones that form the pelvic girdle, 8 bones in the legs, and 52 bones in the feet.

Now there are 5 types of bones based on their shape - long bones, short bones, flat bones, sesamoid bones, and irregular bones.

Long bones are longer than they are wide, and they’re in the limbs and include the humerus, radius, and ulna in the arms; as well as the metacarpals and phalanges of the hand and fingers.

Long bones also include the femur, tibia, and fibula in the leg as well as metatarsals and phalanges of the feet and toes.

During childhood and adolescence, long bones continues to grow and are the bones that are responsible for your height.

Unlike long bones, the short bones have a similar length and width, and that gives them a round or cube-like appearance. They include the carpal bones of the wrist and tarsal bones of the ankle and their main functions are to support the hand and foot.

Flat bones are thin bones, and some of them are curved. They include bones of the skull, the two shoulder blades or scapulae, sternum, and ribs. Their main job is to serve as armor plating that protects vital organs like the brain, heart, and lungs.

Sesamoid bones, are embedded in tendons and they’re like giant sesame seeds - in shape. Most of these bones can be found in the metacarpal phalangeal joints in the hand and metatarsal phalangeal joints in the feet.

The larger sesamoid bones include the pisiform bone that’s in the wrist, and the patella which is the kneecap.

These bones increase the angle between the bone and the tendon of muscles which gives the muscles more leverage. The sesamoid bones also provide support and protects the tendon from wear and tear.

The last type of bones are the irregular bones which are basically the misfits that don’t fit into any of the previous categories. This includes the the facial bones, the mandible, the vertebrae of the vertebral column, and the sacrum and coccyx.

Now, some bones have surface structures that help them function. For example, bones can have tubercles which are small bumps on the bone that serve as an attachment site for muscles.

A large tubercle is called a tuberosity, and an example is the deltoid tuberosity on the humerus which is where the deltoid muscle attaches.

Holes in the bone that allow blood vessels or nerves to pass through are called foramen. An example of that is the foramen magnum in the occipital bone of the skull, which allows the spinal cord to exit the skull.

Bones can also have canals, which are tunnels within the bone that allow structures like blood vessels or nerves to travel through. An example is the optic canal in the sphenoid bone which allows the optic nerve, to travel from the brain to the eyes.

Another name for a canal is a meatus, like the external auditory meatus of the ear, located in the temporal bone which lets sound pass through to the eardrum.

Some bones have a fossa which is a depression within the bone, where another structure rests. One example is the hypophyseal fossa or sella turcica on the sphenoid bone which is like a tiny seat where the pituitary gland rests.

Also, there are sinuses and cavities, which are empty spaces within a bone or formed by multiple bones coming together. Examples include the nasal cavity which is formed by the maxilla, the nasal bone and palatine bone, as well as the paranasal sinuses, like the maxillary sinus, is located within the maxillary bone.